It is a normal metabolite found in human biofluids. (PMID 7693088 , 7688003 ). The phosphoric ester of serine. -- Pubchem. Serine is one of three amino acid residues that are commonly phosphorylated by kinases during cell signalling in eukaryotes. Phosphorylated serine residues are often referred to as phosphoserine. Serine proteases are a common type of protease. - Wikipedia.

This compound belongs to the class of organic compounds known as alpha amino acids. These are amino acids in which the amino group is attached to the carbon atom immediately adjacent to the carboxylate group (alpha carbon).